By law, all you need is proof of rabies shots.Make sure you decalare it at the border crossing.Should be no problem.
Where is the pup coming from, it might e easier to go and pick up?
If its a distance to fly would cost more ,be stressfull and require a regulation size crate,lots of added on fees .You wold be required to go to Canadian Customs or CFIA for inspection and release.

If I buy a dog from the United States, what will I have to do to get the pup back into Canada. Is it easier to drive over and pickup the dog or have the pup delivered to me?

If I where you I would pick it up I am going to Tennesse this coming September and I will pick up my dog. The last one I had the breeder
ship her to me and he flew her and after that all her life she was scared
of loud noise such as motorcyle, snowmobile, air brakes on truck dumping
she would just cower so this time I will pick her up and another thing when
you go through customs be honest with the US agent but here since it's so
Mickey Mouse you can bs them saying that you got the dog for breeding
purposes or rescue or anything you want because guess what you will
pay GST on your dog believe it or not. The last time when I picked up
my dog I went to the airport to pick her up and the custom agent said
"dog returning home" so I didn't say a word and I just signed and took
my dog otherwise I would have paid GST on my dog Taxes on a dog
jeeeeeeeez. Just make sure like the other one said you need the
vet's paper for their shots and as far as I am concerned the bill will disappeared.
thanks

We got our Norwegian Elkhound, Shasta from Tennessee. We declared her at the border..showed them the bill of sale...and they thought she should have cost more...they kept us there for over an hour...checking out dog sites...then I told them to go to www.puppyfind.com and there she was...we didn't have to pay GST because we were gone for over a week. Now after 48 hours you get $400.00 per person exemption.